Method of reclaiming the base of photographic film



United States Patent US. Cl. 260-23 7 Claims ABSTRACT OF THE DISCLOSURE The gelatin layer of photographic film is stripped from the plastic base by immersing the film in a very dilute aqueous solution of potassium carbonate, hypochlorite, and alcohol, the chlorine is reduced in a dilute sodium sulfite 'bath, loosely adhering remnants of the gelatin layer and residual inorganic chemicals are rinsed off with water, and the stripped film base is dried, preferably after dipping in a Water-miscible volatile organic solvent. It is then clean and ready for reuse, for example, in lacquers. The method can be performed in continuous operation.

This application is a continuation-in-part of Ser. No. 677,869, filed Oct. 25, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method for reclaimingthe base of photographic film, and more particularly to the stripping of the gelatin or emulsion layer and associated coatings from photographic films having a synthetic resin base.

The bases or carriers of photographic films are almost exclusively made of synthetic resins which are esters, typically esters of cellulose with acetic, butyric, or nitric acid and polyesters. When films become useless for photographic purposes either before or after exposure and development, they contain valuable materials worth reclaiming, particularly the silver in the emulsion and the plastic base. It is relatively simple to recover the silver values from photographic film, but a simple and practical process for reclaiming the ester material of the film base was not available heretofore. Yet, the film base material can be reused to advantage in lacquers and other organic coatmgs.

The known methods commonly used for removing the gelatin layer and associated secondary coatings rely on very active reagents which tend to saponify the ester material of the film base, particularly cellulose esters with acetic, butyric, or nitric acid, thereby impairing the solubility of the partly deesterified material in lacquer solvents, or which otherwise case degradation of the polymeric material. The only method known to remove gelatin from a plastic base without affecting the latter is based on enzymatic digestion of the gelatin, and is not practical on an industrial scale at this time.

The primary object of the invention is the provision of a practical and inexpensive method which permits removal of the gelatin and associated layers from a photographic film without affecting the film base. More specifically, the invention aims at a method of recovering the clean film base in continuous operation in equipment of reasonable size and cost.

SUMMARY OF THE INVENTION With these objects and others in view, as will become apparent hereinafter, the method of the invention calls "Ice for initial immersion of the film to be stripped in a solution of an alkali metal carbonate having a pH between 8 and 12 and containing an amount of hypochlorite ions corresponding to an active chlorine content of substantially 0.5 to one' percent and 0.5 to 20 percent of a lower alkanol for a time sufiicient to swell and loosen the gelatin layer. Excess chlorine retained by the film is then neutralized by immersion in a dilute aqueous solution of a reducing agent, the loose gelatin and soluble salts are removed by rinsing with water, and the remaining base is dried.

The very dilute reagent solutions employed in the method of the invention do not affect the commonly used film base materials, yet permit practically complete removal of the gelatin layer together with antihalo coatings, protective surface coatings and like materials which may be associated with the gelatin layer.

DESCRIPTION OF PREFERRED EMBODIMENT The following example is further illustrative of the method of the invention, but it will be understood that the invention is not limited there.

EXAMPLE The first tank of a conventional, continuous film developing machine was charged with an aqueous solution of sodium hypochlorite, methanol, and potassium carbonate which was replenished from time to time to keep the concentration of the active ingredients between 0.5 and 1.0 percent active chlorine, 0.5 and 1.0 percent methanol, and enough potassium carbonate to hold the pH between 8 and 12, the actual concentration of potassium carbonate varying between 0.02 and 0.05 percent.

Waste photographic film having mainly cellulose ester bases was conveyed in continuous length through the machine at a speed to make the dwell time in the first tank about 20 minutes whereby the gelatin emulsion layer of the film was swelled, loosened and partly lifted from the base, taking antihalo layers and the like with it.

The film was then passed quickly through an aqueous 0.25% solution of sodium sulfite which neutralized residual chlorine, and then through a vigorous water spray which removed the loosely adhering gelatin and washed water-soluble residual reagents from the film base.

The almost clean base was next passed through methanol to displace the water and dissolve traces of antihalo layer still visible on the film, and was then quickly dried in a stream of air at 50 C.

The film base so recovered was unchanged by the stripping treatment and ready for reuse, for example, in lacquers or similar organic coatings.

The procedure described above is universally applicable to all plastic film bases which are in practical use at this time, and may therefore be used to advantage for mixed batches of film. It may be modified to suit available processing equipment and specific conditions.

Thus, the chlorine content of the first solution may be maintained by introducing chlorine gas which forms active hypochlorite ions as long as the solution is at least slightly alkaline. Alkalinity may be maintained by the use of dissolved alkali metal carbonates other than the preferred potassium carbonate. The corresponding sodium salt is effective although gelatin swells more quickly in the presence of potassium ions. The neutral carbonates may be replaced in part or entirely by acid carbonates, that is, the bicarbonates of sodium or potassium.

Methanol is effective in the first solution mainly because of its ability to enhance wetting of the film even if the film has a protective organic surface coating, thereby hastening attack of the alkaline agents on the underlying gelatin. As little as 0.5% methanol is effective, but the swelling time is reduced by higher concentrations of the alcohol up to about 20%. Methanol cannot be replaced by conventional synthetic surfactants which have been found to cause saponification of the film base in the presence of alkali metal carbonate and hypochlorite. A technical grade of methanol is sufficient for the instant purpose and less expensive at this time than the other lower alkanols which can perform the same function, ethanol, isopropanol and n-butanol being typical of such other alkanols.

The nature and concentration of the reducing agent employed in the second bath to destroy excess chlorine is not critical, but sulfites, hyposulfites, and thiosulfates of the alkali metals are preferred at this time because of low cost, and because they and the products of their reactions with chlorine or hypochlorite and readily removed by a water rinse. They also assist in the removal of silver chloride formed in the first solution in an obvious manner and not in itself relevant to this invention. Sodium sulfite is the cheapest reducing agent of the group and is handled more conveniently than the corresponding bisulfite.

A technical grade of concentrated methanol is the least expensive dehydrating agent suitable for hastening subsequent air drying, but other volatile organic solvents and their mixtures with each other or with minor amounts of water may be substituted if a dehydration step is required at all. If the air drying cycle of the processing apparatus is long enough, the film may be transferred directly from the water rinse to the drier.

The solvent dip or immersion prior to air drying may also remove remnants of antihalo coatings or the like which may survive the preceding treatment if such remnants are objectionable in the intended use of the recovered film base. The solvent composition is chosen so as not to produce significant irreversible changes in the film base.

Suitable solvents other than methanol thus include the other lower alkanols, the lower alkanones, lower alkyl ethers miscible with water, also lower-alkyl loweralkanoates. Acetone, methylethylketone, ethyl acetate and the like may dissolve some of the film base and are therefore employed either for very short periods or in mixtures with other solvents and with water to minimize their dissolving power. If the dehydrating solution contains solvents of relatively low volatility, a final dip in methanol or other more volatile solvent may be used prior to air drying. A'final alkanol dip is preferred after the use of dehydrating solutions containing acetone, methylethylketone, or dioxan.

The following table lists compositions of dehydrating baths which have been used successfully after the water rinse.

Percent Water 10 Methylethylketone 100 Dioxan significant dissolution of cellulose esters from the film base.

Obviously, many other suitable combinations of water miscible organic solvents may be used and will readily be resorted to one the basis of the above teachings, only typical and inexpensive representatives of lower alkanols, lower alkanones, lower alkyl ethers, and lower-alkyl lower-alkanoates having been listed.

The temperature of the several baths employed in the method of the invention is not critical. Operation at ambient temperature is entirely practical and preferred. Dwell times may have to be adjusted at unusually low or high temperatures. The dwell time in the alkaline, alcohol and chlorine bearing first solution may thus be shortened to 10 minutes or less, or lengthened to 30 minutes or more as conditions may warrant, and other changes in operating conditions will readily suggest themselves to those skilled in the art.

I claim:

1. A method of reclaiming the base of photographic film essentially consisting of said base and of a gelatin layer which comprises:

(a) immersing said film in a first aqueous solution of an alkali metal carbonate having a pH between 8 and 12 and containing an amount of hypochlorite ions corresponding to an active chlorine content of substantially 0.5 to one percent and 0.5 to 20 percent of a lower alkanol for a time sufiicient to swell and loosen said gelatin layer;

(b) thereafter immersing said film in a dilute second aqueous solution of a reducing agent for a time sufficient to neutralize excess active chlorine carried by the film;

(c) rinsing said film with water to remove loosened gelatin and water soluble inorganic salts; and

(d) drying the base remaining after said rinsing.

2. A method as set forth in claim 1, wherein said lower alkanol is methanol.

3. A method as set forth in claim 1, wherein said alkali metal is potassium.

4. A method as set forth in claim 1, wherein said reducing agent is an alkali metal salt of sulfurous, hyposulfurous, or thiosulfuric acid.

5. A method as set forth in claim 1, wherein said base prior to said drying and after said rinsing is immersed in a bath mainly consisting of at least one water miscible organic solvent of the group consisting of lower alkanol, lower alkanone, lower alkyl ether, and lower-alkyl loweralkanoate.

6. A method as set forth in claim 1, wherein said base essentially consists of at least one ester of cellulose with acetic, butyric, or nitric acid.

8. A method as set forth in claim 1, wherein said film is immersed in said first solution and said second solution while moving through said solution in continuous length.

References Cited UNITED STATES PATENTS 2,790,728 4/1957 Foster 11763 2,846,334 8/1958 Fleck et al. 117-63 FOREIGN PATENTS 948,619 2/1964 Great Britain 11734 MURRAY TILLMAN, Primary Examiner M. J. TULLY, Assistant Examiner US. Cl. X.R. 11734, 63 

